In oil and gas exploration, directional drilling techniques are sometimes used to control the direction of the drill bit. A rotary steerable drilling system is one type of directional drilling system that allows a drill string to rotate continuously while steering the drill bit to a desired target location in a subterranean formation. Rotary steerable drilling systems are generally positioned at a lower end of the drill string and typically include a rotating drill shaft or mandrel, a housing that rotatably supports the drill shaft, and additional components within the housing that orient the toolface direction of the drill bit at the end of the drill shaft relative the housing. In a normal operating condition, the rotating drill shaft rotates relative to the housing, but there are situations in which it is advantageous to lock the drill shaft to the housing.